Mike,
The backing panels are 1/4" aluminum. Contoured, drilled and tapped to match the Fenders 'n More ally fenders. You can see the bolts in the first picture I posted. The panel was cut with a router. We tried a snowmobile but it left too ragged of an edge.
Lonnie,
KB58 is correct. The black 1/4" steel strap is welded to a 5/16" tab that is attached to the bottom of the UBJ. The UBJ was long enough for it to fit just right. It's also flanged to the upright so it wont turn on it's own.
This design was created (and insisted on) by my mentor, welder, landlord, electrician and occasional assistant, James. He is the owner of the mobil electronics shop where the car was constructed so his thoughts carried a little weight.
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We tried a couple of others but they were way too flimsy. I wanted to do something akin to what Bill Bernard (Tailwind) and Chuck Schaefer (RX7locost) did but James lobbied so much for the 'spat' looking thing that I relented. And it's not very heavy. Plus I think it looks kind of cool.
All the steel runs are short and to date show no signs of fatigue. I check the steel pieces and welds regularly. The aluminum supports are in great shape so far.
I keep thinking I'll be carrying a fender home in the cockpit one day but not yet.